Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

Tunable Metamaterials: The electromagnetic response of metamaterials gives rise to exciting phenomena such as cloaking, negative refraction, and perfect lensing. Their response, however, tends to depend strongly on resonant effects, thereby limiting the application bandwidth. Driscoll et al. (p. 1518 , published online 20 August) combine a split ring resonator array with the phase change material, VO ₂ , to form a metamaterial in which the response can be tuned. The heat-induced phase change of VO ₂ from an insulator to a metal alters the response of split-ring resonator, and, because it displays a hysteresis, the device can retain a “memory” of the induced change. The results may lead to a flexible method for achieving metamaterials operating over a wide bandwidth and to novel switching applications.

Memristors are passive circuit elements which behave as resistors with memory. The recent experimental realization of a memristor has triggered interest in this concept and its possible applications. Here, we demonstrate memristive response in a thin film of vanadium dioxide. This behavior is driven by the insulator-to-metal phase transition typical of this oxide. We discuss details of this form of phase-change memristance and potential applications of our device. Most importantly, our results demonstrate the potential for a realization of memristive systems based on phase-transition phenomena.

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of ${\mathrm{VO}}_2$ does not occur simultaneously with the first-order metal-insulator transition (MIT) near $68°\mathrm{C}$. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted hole excitation, which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate nonequilibrium state.

We demonstrate a metamaterial device whose far-infrared resonance frequency can be dynamically tuned. Dynamic tuning should alleviate many bandwidth-related roadblocks to metamaterial application by granting a wide matrix of selectable electromagnetic properties. This tuning effect is achieved via a hybrid-metamaterial architecture; intertwining split ring resonator metamaterial elements with vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ -a material whose optical properties can be strongly and quickly changed via external stimulus. This hybrid structure concept opens a fresh dimension in both exploring and exploiting the intriguing electromagnetic behavior of metamaterials.